The invention relates to controlling the decarburization of steel components in a furnace during heat treating processes, such as annealing.
During annealing, the metal is heated and then cooled (sometimes repeatedly), usually for the purpose of softening the metal and making it less brittle, therefore, facilitating subsequent cold forming operations. Annealing is often done in batch furnaces, typically bell furnaces or box furnaces. Annealing of carbon-containing steel elongated products (such as wires and rods) is typically done before and after drawing. During drawing, the metal is spread, elongated, pulled, or stretched, for example.
Heat treating processes may be done under different atmospheres in order to minimize scale. Scale is an oxide layer that forms on the iron metal surface during the heating process. Examples of iron oxides include FeO, Fe2O3, and Fe3O4.
In addition to scale, decarburization also may occur under different atmospheres. During decarburization, the metal loses carbon to the atmosphere. One skilled in the art will appreciate the variety of types of steel that may be heat treated, as well as the different uses for heat-treated steel. For any particular type of steel or use, there might be a specific amount of decarburization that is allowable during the heat treating process, or it may be that no decarburization is allowed (xe2x80x9czero decarburizationxe2x80x9d). The allowable limit of decarburization is known as a decarburization specification. For instance, if carbon-containing steel loses carbon during the heat treating process, then the steel will become softer than it was prior to the heat treating process. This may or may not be acceptable to the customer or the intended use for the steel during future tooling.
In addition to decarburization, recarburization may also result from different atmospheres. During recarburization, the metal gains carbon during the heat treating process.
Naturally, the simplest option is to use ambient air as the heat-treating atmosphere within the furnace. However, particularly if the metal being annealed is iron-based, heavy scaling will occur. Decarburization can also occur.
An exogas may also be used for the furnace atmosphere. Exogas is a gas that is generated by an exothermic generator via the combustion of hydrocarbon (typically natural gas) and air. Exogas composition typically contains approximately 2-15% H2, 1-8% CO, and the balance is N2 and impurities such as CO2 and H2O. This atmosphere may provide protection against oxidation (scale), but heavy decarburization can occur.
An endogas is another option for the furnace atmosphere. Endogas is a gas generated by an endothermic generator via a catalytic reaction of hydrocarbon (typically natural gas) and air. Endogas composition typically contains approximately 40% H2, 20% CO, and the balance is N2 and impurities such as CO2 and H2O. Endogas is a powerful reducing atmosphere, where H2 and CO act as scavengers of O2, CO2, and H2O (oxidizing and decarburization species). However, H2 and CO will also react with metal oxides in the furnace (load, stems, radiant tubes, etc.) to generate moisture, which can, in turn, contribute to decarburization. In addition, endogas composition may fluctuate for many of the following reasons. The natural gas composition may vary. There may also be natural gas/air ratio drift. Catalyst performance may also change over time. Recarburization and decarburization can also be difficult to control, because the furnace temperature also varies during the thermal cycle of a batch furnace.
Nitrogen alone is also an option to use as a furnace atmosphere. However, since nitrogen does not react with decarburizing species such as CO2 and H2O, if CO2 and H2O are present in sufficient quantities, decarburization can still occur. Pure nitrogen is used on low carbon steel where decarburization specifications are not too tight. In other words, in low carbon steel, greater decarburization may be acceptable to the customer or to the intended use for the steel.
Another furnace atmosphere may combine nitrogen and endogas. Recarburization and decarburization of the metal may occur in this atmosphere for various reasons. The endogas composition may fluctuate over time. In addition, the reducing species of the endogas may react with metal oxides in the furnace to generate a high dew point. Because nitrogen does not react with H2O, decarburization may result. Also, the furnace temperature may vary during the thermal cycle of a batch furnace, and this makes it difficult to control recarburization and decarburization.
Yet another furnace atmosphere combines nitrogen and a hydrocarbon. Since nitrogen alone does not protect against decarburizing and oxidizing species (H2O and CO2), quantities of hydrocarbon (methane, propane, or propylene for instance) are added in order to generate reducing species which are going to react with H2O and CO2. However, reducing species also react with metal oxides, generating moisture which, in turn, can contribute to decarburization. Chemical reactions involving hydrocarbons at temperatures typically encountered for elongated product annealing (1275xc2x0 F.-1600xc2x0 F.) have slow kinetics. Slow kinetics means that thermodynamic equilibrium is not reached within reasonable short times. If thermodynamic equilibrium is not reached, this means that the equations valid at equilibrium cannot be used for controlling the atmosphere. Therefore, the addition of hydrocarbons makes it difficult to regulate recarburization and decarburization.
For all of these various furnace atmospheres, the gas flow rate is typically kept below or at about 1 renewal per hour. One renewal per hour equals a gas flow sufficient to replace the furnace atmosphere within an hour. Consequently, the actual flow rate of the gas into the furnace chamber depends on the size of the furnace chamber. Flow rate per hour is equal to: (the number of renewals per hour) times (the internal volume of the furnace). For instance, a flow of 1,000 cubic feet per hour of gaseous N2 into a 1,000 cubic foot furnace would give a flow rate of 1 renewal per hour.
When the gas flow rate is low, then the atmosphere in the furnace may lose its positive pressure. Positive pressure means that the pressure inside the furnace is greater than the pressure of the ambient air. When there is positive pressure inside the furnace, then the furnace atmosphere will tend to leak out of the furnace. Conversely, when the furnace atmosphere does not maintain a minimum pressure, then the ambient air will tend to enter the furnace. As a result of ambient air entering the furnace, the oxygen that is in the ambient air may cause oxidation and decarburization to the metal.
Briefly stated, the invention is an apparatus and a method for controlling the decarburization of a steel component during heat treating in a furnace.
In accordance with a method aspect, the method includes heat treating at least one batch of steel components, injecting an inert gas into furnace, and measuring and recording the concentration of CO2 or CO. For the sake of brevity, the abbreviation of xe2x80x9cCO2/COxe2x80x9d shall mean xe2x80x9ceither CO2 alone CO alone or CO2 and CO combined.xe2x80x9d The concentration of CO2/CO at various times during the process gives a CO2/CO concentration profile. The CO2/CO concentration profile is used during the heat treating of subsequent batches of components. An inert gas (such as N2) is injected into the furnace at a flow rate of at least about 2 renewals per hour at predetermined times during the process, correlated to elevated concentrations in the CO2/CO concentration profile.
xe2x80x9cElevated concentrationsxe2x80x9d are preferably defined in relation with the decarburization specification. If a batch of steel components is processed in a given furnace under given conditions (load, temperature, flows, grade, etc.) and if, after treatment, it does not meet the given decarburization specification, then the CO/CO2 concentration recorded during the process is, a posterior, considered as xe2x80x9celevated concentrationsxe2x80x9d relative to the given decarburization specification. If another batch of steel components is processed in a given furnace under given conditions and if it does meet the expected decarburization specification, then the CO/CO2 concentration recorded during the process are not, a posteriori, considered as xe2x80x9celevated concentrationsxe2x80x9d for the given decarburization specifications. Predetermined times for increasing the injection rate of the inert gas are determined, a posterior, (i.e., after processing at least one batch of steel components) as moments in the thermal cycle at which the CO2/CO concentrations previously have been identified as xe2x80x9celevated concentrations.xe2x80x9d
In accordance with a further method aspect, the method includes an analyzer to monitor the CO2/CO concentration in the furnace during the heat treating process. It further includes injecting inert gas (such as N2) in response to a signal from the analyzer. At elevated CO2/CO concentrations, the inert gas is injected into the furnace at a flow rate of at least about 2 renewals per hour.
In accordance with an apparatus aspect, the apparatus includes an analyzer to monitor the CO2/CO concentration in the furnace. It further includes a gas injector, which, responsive to a signal from the analyzer indicating an elevated CO2/CO concentration, injects inert gas into the furnace at a flow rate of at least about 2 renewals per hour.